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Seldomly

Seldomly is an unnecessary variant of seldom. Seldom is already an adverb, so adding the adverbial -ly doesn’t change its meaning. Using seldomly is not a serious error, however. Your spell check probably catches it, and most major dictionaries either don’t list the word or list it as obsolete, yet to many English speakers, seldom without the -ly just doesn’t feel right in certain situations (a similar effect is seen with thusly, an unnecessary variant of the adverb thus). If you are an English speaker and you think seldomly sounds better than seldom in a sentence you’re writing, there is no reason to go against your instincts.

This is not a case where the adverbial suffix has been dropped from a word that once ended in -ly. Seldom goes back to Old English (and through the centuries it was variously spelled seldun, seldenn, seldon, seildunn, and seeldum, until the modern spelling was settled around the 18th century), and it has always been an adverb. Seldomly first appeared many centuries after seldom had already been in use, and it was never the standard form. Still, in this case even the superfluous form is many centuries old.

For a site dedicated to the use of proper grammar, the paragraph explaining seldom and seldomly is poorly written. The paragraph is written in such a way that it can be misconstrued that the word “seldomly” came “many centuries” after the spelling of “seldom” was decided some time in the 18th century. The word “many” isn’t necessarily the most appropriate word to use.

The spelling “seldom” was first used around the 12 century with the first known use of “seldomly” several centuries later in the 16th century.

While the word “seldomly” is viewed as incorrectly because it duplicates the word “seldom”, its use became popular in the latter part of the 20th century because it was more harmonious in certain sentences. Adding “ly” to the end of “seldom” for flow is really no different than interchanging the “e” sound at the beginning of “either” or “neither” for the “i” sound at the beginning of the same words.

As with most unnecessary words, it’s situational. In the above examples, I feel the Sunday Observer and Collegiate Times quotes would be better with seldom.
To me, seldom sounds better if it’s before the verb it is modifying. Seldomly usually sounds better at the end of a sentence or after a verb it is modifying. Despite the fact that seldom would be more correct in either instance.